Burst insertion apparatus for SECAM-PAL transcoder

ABSTRACT

In a SECAM-PAL transcoder, color-difference signals, sequentially recovered from received SECAM signals by a single FM discriminator, are supplied via a first emitter-follower to a first modulating signal input terminal of a balanced modulator, which develops a quasi-PAL signal output using PAL subcarrier frequency oscillations. A DC potential, supplied to a second modulating signal input terminal of the modulator via a second emitter-follower, matches discriminator output level under no-color signal conditions. A third emitter-follower, responsive to a blanking waveform, effects cutoff of the first emitter-follower during blanking periods and delivers elevated blanking pedestal to the first terminal. A fourth emitter-follower, responsive to the same blanking waveform, effects cutoff of the second emitter-follower during blanking periods and delivers matching blanking pedestal to the second terminal. During a portion of the blanking period, a burst gating pulse supplied via the third emitter-follower causes departure from the blanking pedestal level at the first terminal without disturbing the blanking pedestal level at the second terminal, to develop a burst component of controlled magnitude in quasi-PAL signal output.

The present invention relates generally to apparatus for transcoding achrominance signal of a SECAM format to a chrominance signal of a formsuitable for further processing by standard PAL-format chrominancesignal decoder apparatus, and particularly to such transcoder apparatusincorporating an advantageous system for insertion of periodic bursts ofoscillations of PAL subcarrier frequency.

In U.S. Pat. No. 4,233,622, transcoder apparatus is disclosed wherein aSECAM-encoded chrominance signal is converted to a chrominance signal ofa quasi-PAL form for further processing in standard PAL decoderapparatus. The transcoder includes an FM detector serving to demodulaterespective SECAM subcarriers sequentially, and a modulator wherein thecolor-difference signals recovered by the FM detector amplitude modulatein appropriate sequence quadrature-related phases of PAL subcarrierfrequency oscillations derived from the PAL decoder. This signalconversion approach follows the general principles taught in U.K. Pat.No. 1,358,551.

In the transcoder apparatus of the aforesaid U.S. patent application,the color-difference signals supplied to the modulator are augmented byline rate pedestals effecting the insertion of line retrace intervalbursts of PAL subcarrier frequency oscillations in the quasi-PAL signaldeveloped by the modulator. The phasing of the inserted burstsalternates between a first phase corresponding to the oscillation phasesubject to modulation by R-Y color-difference signals and a second phasediffering from the first phase by 180°. Insertion of bursts of the firstphase immediately precedes the development of R-Y modulated subcarrierwaves by the modulator, while insertion of bursts of the second phaseimmediately precedes the development of B-Y modulated subcarrier wavesby the modulator.

The present invention is directed to novel circuit arrangements whichmay be advantageously employed for effecting the aforementioned burstinsertion in the quasi-PAL signal output of the transcoder.

In transcoder apparatus constructed pursuant to an illustrativeembodiment of the present invention, the output of the SECAM FMdiscriminator, after suitable filtering and de-emphasis, passes througha blanking and burst insertion circuit which performs the followingfunctions: (a) insertion of a blanking pedestal; (b) insertion of aburst gating pulse on the blanking pedestal. The output of the insertioncircuit is applied to one of the modulating signal inputs of a balancedmodulator. Modulator balance is achieved by applying an appropriate"modulator balance voltage" to the complementary modulating signal inputof the modulator. This "modulator balance voltage" desirably equals themodulating signal voltage applied to the first input of the modulatorunder no-color conditions, (i.e. if the FM input signal is not modulatedand thus equals a SECAM subcarrier resting frequency). The "modulatorbalance voltage" is derived from a so-called "dummy discriminator",which produces a DC voltage which substantially equals under varyingconditions the no-color DC level of the FM discriminator output (despitesupply voltage fluctuations and/or temperature variations). In order toget modulator balance during blanking, the blanking pedestal is alsoadded to the "modulator balance voltage". However, the burst gatingpulse only appears on the blanking pedestal of the modulating signalapplied to the first input of the modulator; it is not also added to the"modulator balance voltage". Therefore a modulator unbalance occursduring the presence of the burst gating pulse, resulting in a carrierburst at the modulator output.

In the accompanying drawing:

FIG. 1 illustrates, partially schematically and partially by blockrepresentation, a SECAM-PAL transcoder incorporation burst insertionapparatus embodying principles of the present invention; and

FIG. 2 illustrates schematically circuitry which may be employed toimplement the balanced modulator function in the transcoder apparatus ofFIG. 1, in accordance with an illustrative embodiment of the presentinvention.

In the apparatus of FIG. 1, a composite video input signal (derived fromthe video detector, not illustrated, of the color television receiver inwhich the illustrated transcoder is employed) appears at an inputterminal I, and is supplied to a bandpass filter 11. Filter 11 has apassband which encompasses the frequency band occupied by thechrominance signal of a SECAM transmission, and is provided with abandpass characteristic of a "cloche" or bell-shaped form, appropriatelycomplementary to the characteristic employed for high frequencypre-emphasis of subcarrier sidebands in SECAM signal formation.

A limiter 13 accepts the output of bandpass filter 11, and delivers alimited version thereof to an FM discriminator 15, illustratively of thequadrature detector type, as described, for example, in U.S. Pat. No.4,232,268. A tuning control circuit 29 is associated with the FMdiscriminator 15 so as to alter the effective center frequency of itsfrequency discriminator characteristic in a controlled manner, to bedescribed in detail subsequently, which permits sequential demodulationof the respective R-Y and B-Y subcarrier waves by the singlediscriminator.

The output of the discriminator 15, appearing at terminal D, is suppliedto the base electrode of an NPN transistor 16, disposed as anemitter-follower, with its collector electrode directly connected to thepositive terminal (B+) of a DC supply, and its emitter electrodeconnected via emitter resistor 17 to the negative supply terminal(ground). The output of emitter-follower transistor 16 is supplied via alow pass filter 18 and a series resistor 19 to a demodulated signaloutput terminal V. Series resistor 19 cooperates with the seriescombination of resistor 20 and capacitor 21, coupled between terminal Vand ground, to form a de-emphasis circuit, with parameters selected toprovide a frequency response characteristic complementary to themodulating signal pre-emphasis characteristic employed in SECAM signalformation.

Signals appearing at terminal V are supplied to an identification system23, comprising SECAM identification circuitry which monitors the outputof discriminator 15 to determine the correctness or incorrectness of thesequence of operations therein, and effects adjustment of the operationof the aforementioned tuning control cirucit 29 when sequence correctionis required. Illustratively, identification system 23 and the associatedtuning control circuit 29 cooperate in the manner described in thecopending U.S. Patent Application Ser. No. 020,942. For an understandingof the operation of the identification system in such an arrangement, itis appropriate to first consider the operation of the associated tuningcontrol circuit.

For the single FM discriminator 15 to sequentially develop R-Y and B-Ycolor-difference signals at terminal V, it is desired that its centerfrequency tuning be appropriate for demodulation of the R-Y SECAMsubcarrier (4.40625 MHz.) during the image portion of an R-Y lineinterval, and appropriate for demodulation of the B-Y SECAM subcarrier(4.250 MHz.) during the image portion of a B-Y line interval of thereceived SECAM signal. Accordingly, tuning control circuit 29 isarranged to effect a line-to-line switching of the center frequencytuning in response to half line rate control waves applied thereto; thehalf line rate control waves are derived from an output terminal FF of aflip-flop 25 triggered by line rate pulses from a burst gating pulsesource 27. If the flip-flop phasing is correct, this will result intuning appropriate for R-Y subcarrier demodulation during image portionsof R-Y line intervals, and tuning appropriate for B-Y subcarrierdemodulation during image portions of B-Y line intervals; however, ifthe flip-flop phasing is incorrect, the result will be inappropriatecenter frequency tuning for the respective line interval image portions.

To aid in identification of such incorrect phasing when it occurs, linerate pulses of burst interval timing from terminal BG of the burstgating pulse source 27 are utilized in combination with the half linerate control waves from terminal FF to effect a composite control of thecenter frequency tuning, whereby the timing of the changes in centerfrequency tuning is such that, during the lead-in bursts of the SECAMsignal preceding the image portions of both R-Y and B-Y line intervals,the center frequency tuning is the same (e.g., tuned for a centerfrequency corresponding to the R-Y subcarrier).

As a consequence of holding the same (R-Y) center frequency tuning forall burst intervals, pulses appear in the demodulated signal at terminalV during those alternate line interval blanking portions when thelead-in burst frequency deviates from the R-Y center frequency (i.e.,during each lead-in burst occurrence preceding a B-Y line interval).Such pulses are not developed during the intervening line intervalblanking portions when the lead-in burst frequency is equal to the R-Ycenter frequency (i.e., during each lead-in burst occurrence precedingan R-Y line interval).

Illustratively, in the identification system 23, the demodulated signalsappearing at terminal V are applied to a pair of sample-and-holdcircuits. Using respective half line rate control waves of mutuallyopposite phase (derived from terminals FF and FF of flip-flop 25) andcommon line rate gating pulses of burst interval timing (derived fromterminal BG of source 27) for control of the sampling times of therespective sample-and-hold circuits, one sample-and-hold circuit effectsa sampling of the demodulated signals during the lead-in burstoccurrence of alternate line intervals; while the other sample-and-holdcircuit effects a sampling of the demodulated signals during the lead-inburst occurrence of the intervening line intervals. Comparison of theoutputs of the two sample-and-hold circuits in a voltage comparatoryields an output at terminal R, which is indicative of the correctnessor incorrectness of the flip-flop phasing and which is coupled to areset input of the flip-flop circuit 25. When the output is indicativeof incorrect flip-flop phasing, the flip-flop is shut down and thenallowed to restart, whereupon a new comparison is effected, with such aprocess repeated, if necessary, until correct flip-flop phasing isachieved.

With correct phasing of the operation of flip-flop 25, discriminator 15operates properly to form a demodulated signal output at terminal Vwhich comprises image-representative R-Y color-difference signalinformation during the image portions of alternate ones of a successionof line internvals, and image-representative B-Y color-difference signalinformation during the image portions of the intervening ones of saidsuccession of line intervals. For formation of the quasi-PAL signaloutput of the transcoder, a signal path is provided for delivery of thecolor-difference signals appearing at terminal V to a modulating signalinput terminal (M) of a balanced modulator 35. This signal path isformed by a level shifting network comprising an emitter-followerutilizing an NPN transistor 30 disposed with its base electrode directlyconnected to terminal V, with its collector electrode directly connectedto the B+ terminal, and with its emitter electrode returned to groundvia the series combination of resistors 31 and 33, with the junction ofresistors 31, 33 directly connected to modulator terminal M.

The color-difference signals recovered by the SECAM subcarrierdemodulating action of discriminator 15 appear at terminal Vsuperimposed upon a DC component, corresponding to the DC level at thediscriminator output under no-color signal condition (e.g., occurringwhen SECAM signal input is at the appropriate resting frequency) astranslated via the intervening circuitry to terminal V. To ensureproperly balanced operation of modulator 35, whereby subcarrier waveoutput development is precluded under no-color signal conditions (as isappropriate for the quasi-PAL output signal format), the complementarymodulating signal input terminal (M') of balanced modulator 35 iscoupled to the output of a second level shifting network (70, 71, 73),matching the level shifting network (30, 31, 33) associated withterminal M, and responsive at its input to a DC potential closelymatching the aforementioned DC component developed at terminal V.

The second level shifting network employs an NPN transistor 70 disposedas an emitter-follower, with its collector electrode directly connectedto the B+ terminal, and its emitter electrode returned to ground via theseries combination of resistors 71 and 73, with the junction ofresistors 71, 73 directly connected to terminal M', and with theresistance values of resistors 71, 73 matching the respective resistancevalues of resistors 31, 33.

The input to the second level shifting network is supplied via a seriesresistor 63 (matched in resistance value with the series resistor 19 ofthe de-emphasis network 19, 20, 21) connected between the base electrodeof emitter-follower transistor 70 and the emitter electrode of an NPNtransistor 60. Transistor 60 is also disposed as an emitter-follower,with its collector electrode directly connected to the B+ terminal, andits emitter electrode returned to ground via an emitter resistor 61(matched in resistance value with emitter resistor 17). The networkformed by elements 60, 61, 63 is designed to exhibit a DC translatingcharacteristic matching the DC translating characteristic of the networkformed by elements 16, 17, 18, 19. In this connection, it may be assumedthat the design of low pass filter 18 (not shown in schematic detail) iscompatible with achievement of such matching (i.e., by effecting itsfiltering function without DC level shift introduction).

A DC potential input for the base electrode of transistor 60, closelymatching the DC level attained by the discriminator output at terminal Dunder no-color signal conditions, is supplied by a "dummydiscriminator." The "dummy discriminator" function is served by an NPNtransistor 40 disposed as a common-emitter DC amplifier, with itscollector electrode connected via a load resistor 45 to the B+ terminalits emitter electrode returned to ground via emitter resistor 43, andits base electrode connected by resistor 41 to the output terminal B ofa bias source. Illustratively, the bias source, which additionallyserves to supply biasing to discriminator 15, comprises anemitter-follower formed by NPN transistor 50, with its collectorelectrode directly connected to the B+ terminal, its emitter electrodereturned to ground via an emitter resistor 51, and its base electrodeconnected to a point of +1.9 volt potential. The circuit parameters ofthe "dummy discriminator" are chosen so as to supply an output voltagewhich equals the no-color DC level of the discriminator output atterminal D under varying temperature and supply voltage conditions.

During the image portions of the successive line intervals whencolor-difference signals appear at terminal V, the base-emitter paths oftransistors 30 and 70 are forward biased, enabling the respective signalpaths for application of the color-difference signals to modulatorterminal M, and application of the modulator balance voltage derivedfrom the "dummy discriminator" to modulator terminal M'.

Balanced modulator 35 has a pair of carrier wave input terminals C, C,which are driven in push-pull by waves of PAL subcarrier frequencydeveloped in a subcarrier phase switching circuit 37 from referenceoscillations derived from the receiver's PAL decoder apparatus. Thephasing of the supplied waves is altered pursuant to a predeterminedsequence, as controlled by switching control waves supplied to switchingcircuit 37 from terminals FF and FF of flip-flop 25, and terminal BG ofburst gating pulse source 27.

Illustratively, pursuant to an approach described in aforementioned U.S.Pat. No. 4,233,622, the operation of the subcarrier phase switchingcircuit 37 is carried out in such a way that the following results areobtained:

(A) During the image portion of a line interval when B-Ycolor-difference signals are supplied to terminal M, the subcarrierwaves delivered to terminal C are of a first phase;

(B) During the image portion of a line interval when R-Ycolor-difference signals are supplied to terminal M, the subcarrierwaves delivered to terminal C are of a second phase, leading the firstphase by 90°;

(C) During appearance of a burst gating pulse at terminal BG, in theblanking period immediately preceding delivery of R-Y color-differencesignals to terminal M, the subcarrier waves delivered to terminal C areof said second phase; and

(D) During appearance of a burst gating pulse at terminal BG, in theblanking period immediately preceding delivery of B-Y color-differencesignals to terminal M, the subcarrier waves delivered to terminal C areof a third phase, differing from said second phase by 180° and laggingsaid first phase by 90°.

Alterations of the phasing of the subcarrier waves delivered to terminalC take place in a manner maintaining the anti-phasal relationshipbetween the respective carrier wave inputs to modulator 35.

During period (A) above, PAL subcarrier frequency oscillations of thefirst phase appear at the modulator output terminal O, subject toamplitude modulation in accordance with the B-Y color-difference signalinformation recovered by discriminator 15. During period (B) above, PALsubcarrier frequency oscillations of the second phase appear at themodulator output terminal O, subject to amplitude modulation inaccordance with the R-Y color-difference signal information recovered bydiscriminator 15. During scanning of uncolored image regions, when theSECAM subcarrier signals remain at their resting frequencies,oscillations disappear from output terminal O due to the balancedrelationship between the signal levels at the respective modulatingsignal input terminals M, M'. The output signal appearing at terminal O,of the quasi-PAL form disclosed in the aforementioned U.K. Pat. No.1,358,551, is supplied as a chrominance signal input to the receiver'sPAL decoder apparatus.

The emitter-collector path of the discriminator output translatingtransistor 30 is shunted by the emitter-collector path of an NPNtransistor 90, with the collector electrode of transistor 90 directlyconnected to the B+ terminal, and the emitter electrode of transistor 90directly connected to the emitter electrode of transistor 30. Similarly,the emitter-collector path of the "dummy discriminator" outputtranslating transistor 70 is shunted by the emitter-collector path of anNPN transistor 100, with the collector electrode of transistor 100directly connected to the B+ terminal, and the emitter electrode oftransistor 100 directly connected to the emitter electrode of transistor70.

A pair of resistors 85, 87 are connected in series between the baseelectrodes of transistors 90 and 100. A resistor 83 is connected betweenground and the junction of resistors 85, 87. An NPN transistor 80 isdisposed as an emitter-follower, with its collector electrode directlyconnected to the B+ terminal, its base electrode connected to a blankingwaveform input terminal BL (and connected via a resistor 81 to the B+terminal), and its emitter electrode directly connected to the junctionof resistors 85, 87.

During the line interval image portions when image-representativecolor-difference signals appear at terminal V, the blanking waveformappearing at terminal BL swings sufficiently low that transistor 80 iscut off. In the absence of conduction by transistor 80 during thoseperiods, the base-emitter paths of transistors 90, 100 arereverse-biased so that transistors 90 and 100 are held off. However,during the periods intervening such color-difference signal appearances,the blanking waveform at terminal BL swings high, rendering transistor80 strongly conducting, with the consequence that transistors 90 and 100are turned on. Conduction by transistors 90 and 100 during theseintervening periods is such as to elevate the potentials at the emitterelectrodes of transistors 30 and 70 to a level rendering transistors 30and 70 nonconducting. Under these circumstances, the signal pathsnormally supplying the outputs of discriminator 15 and the "dummydiscriminator" to the respective modulator terminals M, M' aredisrupted. Instead, terminals M and M' receive elevated blankingpedestals of matching magnitude, supplied by the respective transistors90 and 100 via the matched dividers 31, 33 and 71, 73. Duringundisturbed presence of matching pedestals at the respective terminalsM, M', oscillation appearance at output terminal O is precluded.

To provide the desired burst components for the quasi-PAL signal outputat terminal O, during the periods (C) and (D) discussed above, burstgating pulses appearing at terminal BG are supplied to the baseelectrode of transistor 90. During each burst gating pulse appearance,terminal M is caused to depart from the aforementioned blanking pedestallevel. With the conducting transistor 80 effectively clamping thejunction of resistors 85, 87 to the potential of the B+ terminal, theapplication of the burst gating pulse to the base electrode oftransistor 90 has no significant effect on the potential at the baseelectrode of transistor 100. Accordingly, during each burst gating pulseappearance, terminal M' does not depart from the aforementioned blankingpedestal level.

The resultant imbalance between the levels at terminals M, M' causesappearance of a burst of PAL subcarrier frequency oscillations ofrespectively appropriate phase at output terminal O during the periods(C) and (D). The magnitude of the burst gating pulse supplied to thebase electrode of transistor 90 is chosen to provide the quasi-PALsignal with a burst component magnitude of a level assuring unkillingaction by the color killer circuits of the PAL decoder apparatus, andadjustment of chrominance signal gain to a desired level by the ACCcircuits of the PAL decoder apparatus.

It will be noted that the cutoff of transistor 30 throughout eachblanking period bars the delivery to terminal M of the pulses developedat terminal V in response to alternate lead-in burst occurrences in thereceived SECAM signal, thus avoiding alternate line disturbances of thedesired magnitude for the burst component of the output quasi-PALsignal.

FIG. 2 illustrates circuitry which may be employed advantageously toimplement the functions of balanced modulator 35 in the FIG. 1arrangement. In FIG. 2, the modulating signal input terminals M, M' arerespectively connected to the base electrodes of respective NPNtransistors 101 and 103. The emitter electrodes of transistors 101 and103 are interconnected by a resistor 104. The collector-emitter paths ofrespective NPN current source transistors 105 and 107 are connectedrespectively between the emitter electrodes of transistors 101, 103 andground.

Bias for the base electrodes of current source transistors 105, 107 issupplied in common from a v_(be) supply (121, 123, 125, 127) of the typeshown in U.S. Pat. No. 3,430,155. The bias supply includes an NPNtransistor 121 disposed as an emitter-follower, with its collectorelectrode directly connected to the B+ terminal, its emitter electrodeconnected to ground via resistor 125, and its base electrode connectedvia resistor 127 to a point of 5.8 volt potential. An additional NPNtransistor 123 is disposed as a common-emitter stage, with its baseelectrode directly connected to the emitter electrode of transistor 121,with its emitter electrode directly connected to ground, and with itscollector electrode directly connected to the base electrode oftransistor 121. A direct connection is provided between the bias supplyoutput terminal (emitter electrode of transistor 12) and the baseelectrodes of current source transistors 105, 107.

A pair of NPN transistors 111, 113 are disposed in a differentialamplifier configuration, with their interconnected emitter electrodesdirectly connected to the collector electrode of transistor 101. Asecond pair of NPN transistors 115, 117 are also disposed in adifferential amplifier configuration, with their interconnected emitterelectrodes directly connected to the collector electrode of transistor103. Oscillations of PAL subcarrier frequency applied to carrier waveinput terminal C are directly supplied in common to the base electrodesof transistors 111 and 115. Oscillations of similar magnitude applied tothe complementary carrier wave input terminal C (in anti-phasalrelationship to the oscillations applied to terminal C) are directlysupplied to the base electrodes of transistors 113 and 117.

The collector electrodes of transistors 111 and 117 are directlyconnected to the B+ terminal. A common load resistor 119 is provided fortransistors 113 and 115, and connected between their interconnectedcollector electrodes and the B+ terminal. The modulator output terminalO is directly connected to the interconnected collector electrodes oftransistors 113 and 115.

In the arrangement of FIG. 2, when the respective modulating signalterminals M, M' are maintained at matching DC levels, a balancecondition exists which precludes subcarrier oscillation appearance atoutput terminal O. However, when the potentials at terminals M and M'are unbalanced, subcarrier oscillations appear at output terminal O withan amplitude dependent upon the magnitude of imbalance.

In an illustrative utilization of the present invention, circuitry ofall of the illustrated elements of the FIG. 1 arrangement, with theexception of filters 11 and 15 and de-emphasis circuit 19, 20, 21, aresubject to realization on a common monolithic integrated circuit chip,and utilization with a power supply establishing a +12 volt potential atthe B+ terminal.

What is claimed is:
 1. Apparatus for converting an image-representativechrominance component of received signals which are encoded in SECAMfashion to a chrominance signal suitable for application to a PALdecoder, said apparatus comprising:frequency discriminator means,responsive to said chrominance information encoded in SECAM fashion, forsequentially developing respective first and second color-differencesignals superimposed upon a common DC component; a source ofoscillations of a frequency corresponding to the standard PAL colorsubcarrier frequency; a balanced modulator having a pair of carrier waveinput terminals, first and second modulating signal input terminals, andan output terminal; means for applying oscillations from said source inpush-pull to said carrier wave input terminals; means for applying theoutput of said frequency discriminator means to said first modulatingsignal terminal; means for applying a first DC potential matching saidDC component to said second modulating signal terminal; means, operativeduring periodic intervals of said received signals which are free ofsaid image-representative chrominance component, for applying a secondDC potential in common to said first and second modulating terminals,while disabling said discriminator output applying means and said firstDC potential applying means; and means, operative during only selectedsegments of said periodic intervals, for applying a burst gating pulseto only one of said modulating signal terminals.
 2. Apparatus inaccordance with claim 1, wherein:said frequency discriminator outputapplying means includes a first transistor, having base, emitter, andcollector electrodes, disposed as an emitter-follower, with its baseelectrode receiving said discriminator output and its emitter electrodecoupled to said first modulating signal input terminal; said first DCpotential applying means includes a second transistor, having base,emitter, and collector electrodes, disposed as an emitter-follower, withits emitter electrode coupled to said second modulating signal inputterminal; and said second DC potential applying means includes third andfourth transistors, each having base, emitter, and collector electrodes,and each disposed as an emitter-follower, with the emitter electrode ofthe third transistor directly connected to the emitter electrode of saidfirst transistor, and with the emitter electrode of said fourthtransistor directly connected to the emitter electrode of said secondtransistor; said second DC potential applying means also including meansfor applying, in common, to the base electrodes of said third and fourthtransistors, during said periodic intervals, a DC potential of suchmagnitude as to render said third and fourth transistors conducting andsaid first and second transistors nonconducting; and wherein said burstgating pulse applying means includes means for supplying a burst gatingpulse to the base electrode of only one of said third and fourthtransistors.
 3. Apparatus in accordance with claim 2:wherein saidimage-representative chrominance component appears during image portionsof a succession of line intervals, said image portions of said lineintervals following chrominance-free blanking portions thereof, withsaid burst gating pulse applying means operative during respectivesegments of said blanking portions; and wherein said oscillationapplying means includes means for delivering, to a given one of saidcarrier wave input terminals: (1) oscillations of a first phase, duringthe image portions of alternate ones of said succession of lineintervals; (2) oscillations of a second phase, differing from said firstphase by 90°, during the image portions of the intervening ones of saidsuccession of line intervals; (3) oscillations of said first phase,during said blanking portion segments of said alternate line intervals;and (4) oscillations of a third phase, differing from said first phaseby 180°, during said blanking portion segments of said intervening lineintervals.